There is conventionally proposed a hybrid vehicle incorporating an engine and first and second rotating electric machines (motor generator). For example, Japanese Patent Laying-Open No. 10-238380 discloses a hybrid vehicle incorporating an engine, a first rotating electric machine (motor generator), connected to the engine to determine the engine speed, and a second rotating electric machine (motor generator) to determine the driving force of the vehicle.
Particularly, Japanese Patent Laying-Open No. 10-238380 discloses a control apparatus for improving the engine response by setting appropriately the torque command value of the first and second motor generators during transient driving in which the vehicle is increased or decreased in speed. For example, there is disclosed an operation of correcting the torque command value for the first motor generator towards the increasing side and the torque command value for the second motor generator towards the decreasing side during a vehicle speed reducing mode such as an engine fuel cut mode.
Further, Japanese Patent Laying-Open No. 2006-320039 discloses a control configuration of improving the controllability of the motor current in a motor drive system by modifying the motor applied voltage according to the changing ratio of the motor revolution speed when the speed of the AC motor that is the subject of control rapidly changes.
In a hybrid vehicle based on a configuration as disclosed in Japanese Patent Laying-Open No. 10-238380, the operating state of the first motor generator is controlled such that the engine speed is maintained at the high efficiency region to improve the fuel consumption. Therefore, when engine control to reduce the engine output by fuel cut or the like is executed under the state where the output torque is regulated such that increase in the speed of the first motor generator is suppressed, there is the possibility of the revolution speed of the first motor generator dropping rapidly in accordance with the engine inertia when the engine speed is reduced. Particularly, such a problem is apt to occur readily in a hybrid vehicle of a high power output since the engine inertia increases corresponding to a higher engine power.
In a hybrid vehicle, power balance is established such that any insufficiency or excess in the total power of the first and second motor generators (sum of power consumption and/or generated power) is covered by the input/output power of the power storage device such as a battery. It is therefore necessary to control the power balance such that the aforementioned total power is within a predetermined range in order to prevent overcharging and overdischarging at the power storage device as well as to avoid generation of overcurrent at the power converter (inverter, converter, and the like) disposed between the power storage device and the first and second motor generators.
However, if the revolution speed of the first motor generator rapidly drops by the problem set forth above, the rapid reduction in the electrical power generated by the first motor generator cannot be compensated for and the power balance will be disturbed, leading to the possibility of causing overdischarging from the power storage device.